SIZE-DEPENDENT REACTIVITY AND BIOAVAILABILITY OF HEMATITE NANOPARTICLES

Environmental scientists have long recognized that Fe (hydr)oxides play important roles in pollutant sorption, fate and transport and in Fe supply to microorganisms. However, only recently has it been shown that Fe(hydr)oxide mineral structure, stability, and reactivity may change as a function of (nano)particle size. For example, the structure of the oxide mineral hematite may become more similar to that of the hydroxide mineral goethite at particles sizes less than a few tens of nanometers. Our research is investigating the size-dependent bioavailability of hematite nanoparticles to aerobic bacteria. Batch abiotic dissolution experiments using the microbial siderophore desferrioxamine B (DFOB) have revealed fast release of ‘transient’ Fe from hematite nanoparticles < 10 nm, and that < 10 nm particles dissolve about an order of magnitude faster than particles a few tens of nanometers larger. Results of biotic experiments using the aerobic, siderophore-producing soil bacterium Pseudomonas mendocina and a siderophore (-) mutant reveal enhanced growth by P. mendocina WT and mutant on < 10 nm particles (relative to 72 nm particles) under Fe-limited conditions. Experiments with the siderophore(-) mutant show that the enhanced bioavailability may be related to mechanisms that depend on cell/nanomineral proximity. The siderophore(-) bacteria readily acquire Fe from particles < 10 nm but must be in direct physical proximity to the nanomineral. Particles < 10 nm appear to be capable of penetrating the outer cell wall, offering one potential Fe acquisition pathway. Cell-surface-associated molecules and/or processes could also be important, potentially including cell-wall associated reducing capability. The observed increased bioavailability of < 10 nm hematite particles has implications not only for biogeochemical Fe cycling but also for biomedical applications involving engineered nanoparticles.